1. Field of the Invention
The present invention relates to a semiconductor thin film formed on a substrate having an insulating surface, and a semiconductor device having the semiconductor thin film as an active layer. Particularly, the present invention relates to a structure in a case where a material containing silicon as the main ingredient is used for the semiconductor thin film.
2. Description of the Related Art
In recent years, attention is paid to a technique to constitute a thin film transistor (TFT) using a semiconductor thin film (with a thickness of about several hundreds to several thousands xc3x85) formed on a substrate having an insulating surface. A thin film transistor is widely used for electric devices such as an IC or an electrooptical device, and particularly, speedy development is required for a thin film transistor as a switching element for a picture display device.
For example, in a liquid crystal display device, trials are made to apply TFTs to all electric circuits such as a pixel matrix circuit for controlling each of pixel regions arranged in matrix, a driving circuit for controlling the pixel matrix circuit, and a logic circuit (processor circuit, memory circuit, etc.) for processing data signals from the outside.
In the present circumstances, although a TFT using a noncrystalline silicon film (amorphous silicon film) as an active layer has been put into practical use, a TFT using a crystalline silicon film (polysilicon film, polycrystalline silicon film, etc.) is needed for an electric circuit requiring further high speed performance, such as a driving circuit and a logic circuit.
For example, as a method of forming a crystalline silicon film on a glass substrate, techniques disclosed in Japanese Patent Unexamined Publication Nos. Hei. 7-130652 and Hei. 8-78329 by the same assignee as the present application are well known. The techniques disclosed in these publications enable the formation of a crystalline silicon film superior in crystallinity by using a catalytic element for promoting crystallization of an amorphous silicon film and by a heat treatment at a temperature of 500xc2x0 C. to 600xc2x0 C. for about 4 hours.
Especially, the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-78329 causes crystal growth almost parallel to the surface of a substrate by applying the above-mentioned technique, and the present inventors refer to the formed crystallized region especially as a horizontal growth region (or a lateral growth region).
However, even if a driving circuit is constructed by using such a TFT, it still does not completely satisfy the required performance. Especially, under the present circumstances, it is impossible to constitute a high speed logic circuit requiring an extremely high speed operation in such a range of from MHz to GHz by a conventional TFT.
The present inventors have made various trials for improving crystallinity of a crystalline silicon film (called a polycrystalline silicon film) including crystal grain boundaries. Semi-amorphous semiconductor (Japanese Patent Unexamined Publication No. Sho. 57-160121), monodomain semiconductor (Japanese Patent Unexamined Publication No. Hei. 8-139019) and the like can be cited.
The concept common to the semiconductor film disclosed in the above-mentioned publications is to make crystal grain boundaries substantially harmless. That is, the most important object is to substantially eliminate the crystal grain boundaries to cause smooth movement of carriers (electrons or holes).
However, even by the semiconductor film disclosed in the above-mentioned publications, it is insufficient to perform high speed operation required by a logic circuit. That is, in order to realize a system-on-panel having a built-in logic circuit, the development of a completely new material, which has not conventionally existed, has been demanded.
The present invention has been made in response to such a demand, and an object of the present invention is therefore to provide a semiconductor thin film for realizing such a semiconductor device having extremely high performance, as can constitute a high speed logic circuit, which can not be manufactured by a conventional TFT. Another object of the present invention is to provide a semiconductor device using such a semiconductor thin film.
According to an aspect of the present invention, a semiconductor thin film is made of an aggregate of a plurality of rod-like or flattened rod-like crystals containing silicon as the main ingredient, the thin film has substantially plane orientation of {110}, and one kind or plural kinds of elements selected from the group except at least C (carbon), N (nitrogen), O (oxygen), and S (sulfur) exist in the film in addition to silicon.
According to another aspect of the present invention, in the above semiconductor thin film, the element existing in the film in addition to silicon is one kind or plural kinds of elements selected from the group consisting of Ni (nickel), Co (cobalt), Fe (iron), Pd (palladium), Pt (Platinum), Cu (copper), and Au (gold), and the concentration of the element is not larger than 5xc3x971017 atoms/cm3 (or not larger than 0.001 atomic %).
Incidentally, since the concentration of silicon is about 5xc3x971022 atoms/cm3 in a semiconductor film consisting of only pure silicon, the impurity element of 5xc3x971017 atoms/cm3 is equivalent to the existence with a concentration of about 0.001 atomic %. Thus, for example, in a semiconductor thin film in which germanium of several % is contained in silicon, although the expression by xe2x80x9catomic %xe2x80x9d is slightly changed, the absolute concentration of 5xc3x971017 atoms/cm3 is not changed.
In the present specification, the concentration of each element is expressed by using measurement results by SIMS (Secondary Ion Mass Spectroscopy). Incidentally, the concentration of each element contained in a film is defined by the minimum value obtained from SIMS measurement results. However, the concentration in a region where measurement errors are large, such as in a film interface, is not considered as the measurement results.
As described above, the semiconductor thin film of the present invention is characterized in that there are no or substantially no elements of C (carbon), N (nitrogen), O (oxygen), and S (sulfur), which obstruct crystallization. This is a structure which can be achieved by complete impurity (pollution) management.
Although detailed description will be made in embodiments, the semiconductor thin film of the present invention contains silicon as the main ingredient, and its state is transformed from an amorphous state to a crystalline state. At the crystallization, such a catalytic element as described above is used.
However, it has been found through experiments by the present inventors that if at least any one element of C (carbon), N (nitrogen), O (oxygen), and S (sulfur) exists in the film, a bad influence is exerted upon the crystallization mechanism using the catalytic element.
As a typical mixing path of C (carbon), N (nitrogen), O (oxygen), and S (sulfur), the time of film formation of an amorphous film on a substrate can be cited. Thus, it is important to suppress the concentration of these impurity elements at the initial film formation as low as possible (preferably to completely remove) for securing excellent crystallinity. Of course, it is needless to say that care should be paid at any time other than the film formation.
In the present invention, since the mixing of C (carbon), N (nitrogen), O (oxygen), and S (sulfur) at the film formation of an amorphous film is thoroughly avoided, the above-described concentration of the impurities contained in the final semiconductor thin film (for example, in the state where the semiconductor thin film is incorporated in a completed semiconductor device) inevitably becomes very low.
The present inventors consider that it is important to manage such that, in the final semiconductor thin film, the concentration of C (carbon), N (nitrogen) and S (sulfur) is less than 5xc3x971018 atoms/cm3 (less than 0.01 atomic %), and the concentration of O (oxygen) is less than 1.5xc3x971019 atoms/cm3 (less than 0.03 atomic %).
Moreover, the present inventors consider that it is necessary to make preferably such a state that the concentration of C (carbon), N (nitrogen), O (oxygen), and S (sulfur) existing in the final semiconductor thin film is not larger than the detection lower limit of the SIMS analysis, and to make more preferably such a state that any of those elements do not exist at all, in order to obtain superior crystallinity.